Liquid crystal display device with birefringence compensator

ABSTRACT

A display device comprising a liquid-crystal cell element ( 20 ) placed between two polarizers ( 23, 28 ) comprising at least one optical structure for compensating for the variations in birefringence of said liquid crystal according to the viewing angle. The optical compensation structure comprises at least one oblique-axis film ( 25, 26 ) of polymerized liquid-crystal type suitable for at least partly compensating for the undesirable effects of the natural birefringence of the liquid crystal, combined with a volume hologram ( 24, 27 ) of small retardation suitable for improving the compensatability of said film.

[0001] The present invention relates to liquid-crystal display deviceswith compensation of the birefringence, allowing the viewing angle ofthe display device to be considerably increased.

[0002] It applies especially in electrooptic display devices and morespecifically in liquid-crystal displays, used in transmission, inreflection or even in projection on a screen.

[0003] Liquid-crystal screens have experienced very substantial growthwith the development of portable computers using TFT (Thin FilmTransistor) technology and a TN (Twisted Nematic) liquid-crystal cell.

[0004] Most liquid-crystal displays or screens suffer from a majordrawback, namely the limited viewing angle at which they can beobserved: upon deviating from the normal to the surface of the displayor screen, the contrast between black and white decreases considerablyand the image presented deteriorates. This is because, owing to theintrinsic birefringence of the liquid crystal, the contrast level dropsas soon as the observer deviates from the normal to the screen and, forcertain areas of observation, the gray levels are inverted.

[0005] This phenomenon, acceptable for some applications, mustabsolutely be compensated for when it is a question of producingcomputer screens or any display devices that can be consulted by severalobservers at the same time.

[0006] The viewing angle properties of a screen or display of the LCD(liquid-crystal display) type are generally evaluated using a conoscopewhich gives the isocontrast curves as a function of the angle ofobservation, characterized by the following two angles:

[0007] θ=angle of the observer with respect to the normal of the screen;

[0008] φ=angle of projection of the direction of observation in theplane of the screen, with respect to the East-West (horizontal) axis.

[0009]FIG. 1 depicts the conoscope of an uncompensated TN cell. Thisconoscope shows that the range of viewing angles for which the contrastfor example greater than 50 is small.

[0010] The prior art discloses various methods and structures whoseobjective is to remedy the aforementioned problem.

[0011] 1) Multidomains

[0012] A first approach consists in modifying the structure of the cellby creating, in each elementary cell (pixel), several domains in whichthe anchoring of the liquid crystal is different. The averaging effectthus obtained reduces the problem substantially, but leads to anincrease in the complexity in the process for manufacturing the screen.

[0013] 2) Novel Electrooptic Effects

[0014] A second approach consists in using other types of liquid cellsin which the alignment, the nature of the liquid crystal or theaddressing principle are different from a TN (twisted nematic). Some ofthem, such as IPS (In Plane Switching), have resulted in commercialproducts having properties equivalent to that of a TN and possessing alarge viewing angle. However, these cells are based on complex effectswhich are not always under control in the manufacture of LCD screens.

[0015] 3) Birefringent Films

[0016] A third approach does not modify the structure of the cell butcorrects the birefringence of the liquid crystal by adding one or moreoptimized birefringent films in order to compensate for the effect ofthe liquid crystal. The compensation philosophy is the following: theproblem of the viewing angle of liquid-crystal cells stems from thebirefringent character of the liquid crystal, which converts thepolarization of light wave differently according to its angle ofincidence. Since extinction between cross polarizers is only possible ifthe output polarization is linear, black is obtained only for anglesclose to the normal to the screen. The addition of films possessing an“inverse” birefringence makes it possible to reduce, or even eliminate,this birefringence.

[0017] Comments on Birefringent Films

[0018] A birefringent medium is characterized by its index ellipsoid,the surface characteristic of the index of propagation of a lightwave ofgiven polarization and of given direction: the axes of the intrinsicframe of the ellipsoid constitute the intrinsic axes of the medium andthe length of these axes is equal to the index of propagation of thelight polarized along the corresponding axis.

[0019] If the ellipsoid is of circular cross section, the medium is“uniaxial”. For a uniaxial medium, the index along the axis ofrevolution or optical axis is called the extraordinary index n_(e) andthe index along the other two axes is the ordinary index n_(o), asdepicted in FIGS. 2a and 2 b. If the extraordinary index n_(e) isgreater than the ordinary index n_(o), the medium is called a positiveuniaxial medium and the ellipsoid is elongate, in the form of a “cigar”(FIG. 2a). The extraordinary axis is the slow axis. Conversely, if theextraordinary index n_(e) is less than the ordinary index n_(o), themedium is called a negative uniaxial medium and the ellipsoid isflattened, in the form of a cushion or “dish” (FIG. 2b). Theextraordinary axis is the fast axis.

[0020] The difference between these two indices is very small, forexample of the order of 1%, but it is sufficient to introduce very largechanges in polarization.

[0021] The medium is biaxial if the ellipsoid is not a body ofrevolution, that is to say there are three orthogonal intrinsic axeswith three different indices.

[0022] The inclination of the optical axis for a uniaxial medium isindicated by the angles (θ,φ) in which:

[0023] θ is the angle of the optical axis with respect to aperpendicular to the plane of the screen;

[0024] φ is the projection of the optical axis in the plane of thescreen with respect to the east-west direction.

[0025] The retardation R_(o) of a uniaxial film is defined as follows:

[0026] R_(o)=(n_(e)−n_(o))d, where d corresponds to the thickness of thefilm.

[0027] If R_(o)>0, the film is called a positive uniaxial film.

[0028] If R_(o)<0 the film is called a negative uniaxial film.

[0029] Liquid crystals are positive uniaxial media, the optical axiscorresponding to the director of the liquid-crystal molecule.

[0030] There are many combinations of uniaxially or biaxially basedbirefringent films allowing the viewing angle of a liquid-crystal cellin general, and a twisted nematic or TN cell in particular, to beimproved.

[0031] The birefringent films are generally positioned between thepolarizers and the substrates of the cell, in various geometricalconfigurations.

[0032]FIG. 3 shows an example of an arrangement of birefringent filmsfor compensating a liquid-crystal cell in terms of viewing angle.

[0033] The liquid-crystal cell 1 is positioned between a first assemblyformed by a polarizer 10 and by a compensator 11, which consists forexample of a birefringent film, and a second assembly formed by acompensator 12 and by an analyzer 13. The reference 14 denotes thesubstrate of the liquid-crystal cell.

[0034] Various methods exist for obtaining birefringent films, some ofwhich are given below by way of illustration but not at all implying anylimitation.

[0035] Stretched Plastic Film.

[0036] By uniaxially or biaxially stretching a plastic film (PVA, shortfor polyvinyl alcohol, polycarbonate or CTA, short for cellulosetriacetate), it is possible to obtain any “in plane” birefringence(negative uniaxial type, positive uniaxial type, biaxial type), that isto say with the axes of the index ellipsoid lying within the plane ofthe film or along the normal. However, at the present time, thistechnology does not allow inclined optical axes to be obtained. As it isoften necessary to provide birefringent films with optical axis inclinedwith respect to the plane of the film, the performance of thecompensators is limited thereby.

[0037] The retardations obtained by this technology are of the order of−100 nm and more. For some orientations of the optical axis of the film,it is difficult with this method to obtain low birefringence values, andto do so reproducibly, for example for values falling within the [−20,−60 nm] range.

[0038] Oblique Films

[0039] A known effective technique for compensating for thepositive-type birefringence of a nematic liquid-crystal cell consists,for example, in using a negative-type birefringent film. For example, inorder to compensate a TN cell effectively, it is necessary to provide anegative-type film whose optical axis is inclined with respect to theplane of the substrate.

[0040] Fuji Film

[0041] It is known to couple, on each side of a TN cell, a continuumconsisting of an oblique negative uniaxial medium and an “in-plane”positive uniaxial medium generally obtained by stretching a plasticfilm. Such a compensation film is disclosed, for example, in patent U.S.Pat. No. 5,583,679 or in the publication [1] entitled “Application of anegative birefringence film to various LCD modes” by N. Mori et al,Proceedings SID 97, pp 11-88.

[0042] The inclined negative uniaxial medium continuum is obtained usingpolymerized discotic liquid-crystal molecules. This method ofcompensation has given rise to a film sold by Fuji and denoted in therest of the description by “Fuji film”.

[0043] The Fuji-type solution therefore comprises a continuum of anegative uniaxial medium consisting of a splay of polymerized discoticliquid-crystal molecules and of a negative uniaxial medium of opticalaxis perpendicular to the substrate obtained via a plastic (CTA) film,which is also the substrate of the polarizers used in display, asexplained in the aforementioned publication [1].

[0044] In its current commercial version, the Fuji film consists of astack comprising a CTA (cellulose triacetate) and of a layer ofpolymerized discotic molecules, this being depicted in FIG. 4.

[0045] The CTA substrate 20 of the Fuji film is a stretched plastic filmof the negative uniaxial type, the optical axis of which isperpendicular to the plane of the layers. To use the Fuji filmcorrectly, it has to be combined with a polarizer that itself has a CTAsubstrate of well-defined retardation, the CTA of the polarizer and theCTA of the Fuji film being bonded together. The values of the CTAretardations are around −40 nm for example.

[0046] The multilayer recommended by Fuji for obtaining goodcompensation must include a layer of polymerized discotic molecules 21and a negative uniaxial film of optical axis perpendicular to the planeof −80 nm retardation.

[0047] The structure of the layer of polymerized discotic molecules isillustrated schematically in FIG. 4.

[0048] The Fuji film is a compensation film developed for compensating aTN liquid-crystal film having a thickness d of 4.7 μm. Theliquid-crystal molecules become increasingly inclined on going away fromthe polarizer. Their optical axes are initially inclined at an angle αof 4° to the normal to the surface, attaining a final angle ofinclination of 68°. Each film compensates a liquid-crystal half-cell.The compensation of such a liquid-crystal cell therefore requires theuse of a Fuji film placed on each side of the cell.

[0049] The compensation principle is based on the fact that since thediscotic molecules have the reverse birefringence of the nematicmolecules constituting the liquid-crystal cell, each discotic moleculecompensates for a nematic molecule of parallel optical axis.

[0050]FIG. 5 shows the isocontrast curves for a liquid-crystal cellcompensated by a film sold by Fuji. A Fuji-type film is placed on eitherside of the liquid-crystal cell.

[0051] Holography

[0052] Another method for obtaining a negative uniaxial medium is basedon the use of a holographic grating. When the fringe spacing is smallenough compared with the illumination wavelength, the hologram operatesin form birefringence mode and is equivalent to a negative-type uniaxialmedium whose optical axis is coincident with the normal to the fringeplane. Such a correction method is described, for example, in thepatents FR 2 754 609 and FR 2 778 000 or else in the document entitled“TN-LCD viewing angle compensation with holographic volume gratings” byC. Joubert et al., Photonic West '99 SPIE Proceedings, No. 3635, 137-142(1999).

[0053] 4) Improvement of Films

[0054] The properties of the abovementioned films, in particular theFuji film using polymerized discotic molecules, may also be improved byadding a film of small retardation, of around 20 to 50 nm, with a givenorientation. Thus, it is possible to combine the aforementioned Fujifilm with a negative-type film of about −25 nm retardation with theoptical axis lying, for example, in the plane of the substrate. Adifferent orientation of the optical axis is also possible. An exampleof such a structure is given in the article entitled “Improvement of theoptical characteristics of a twisted-Nematic Display using negative inplane and splayed discotic films”, Proceedings of the SID '98, p. 694 byT. A. Sergan and J. R. Kelly. This article recommends the use of astretched film in the plane having an optical axis lying within theplane of the screen.

[0055] The concept of the invention consists especially in combining avolume hologram with a Fuji-type compensation film so as to improve thecompensatability of the film. In particular, the characteristics of thevolume hologram are chosen depending on the film whose compensatabilityis to be improved.

[0056] The invention considers an optical axis of any inclination forthe film.

[0057] The invention also opposes, in addition to using a holographicfilm, to optimize the CTA of the Fuji film, the combination of the twoimprovements (holographic film+CTA optimization) very substantiallyimproving the performance of the Fuji film.

[0058] The present invention relates to a display device comprising aliquid-crystal cell element placed between two polarizers, comprising atleast one optical structure for compensating for the variations inbirefringence of said liquid crystal according to the viewing angle. Itis characterized in that said optical compensation structure comprisesat least one oblique-axis film of polymerized liquid-crystal typesuitable for at least partly compensating for the undesirable effects ofthe natural birefringence of the liquid crystal, combined with a volumehologram of small retardation suitable for improving thecompensatability of said film.

[0059] The oblique-axis film may be of the nematic or polymerizeddiscotic liquid-crystal type.

[0060] The hologram is, for example, a holographic film having anoptical axis in the plane of the oblique-axis film or else a holographicfilm having an optical axis tilted with respect to the plane of thisfilm.

[0061] The value of the retardation of the said volume hologram,expressed in absolute value, is for example less than −150 nm,preferably between −10 and −100 nm.

[0062] The holographic assembly may consist of at least two holographicfilms each comprising a multilayer grating the layers of which havetheir own orientation.

[0063] According to one embodiment, the oblique-axis film comprises, forexample, a stretched plastic film of the CTA type having a negativeuniaxial birefringence and in that the retardation value of this film istailored to the holographic film and to the oblique-axis film.

[0064] The liquid-crystal cell is, for example, of the twisted nematictype.

[0065] The device according to the invention is used, for example, tocompensate for the birefringence effects in display devices such asmicrocomputer screens.

[0066] The invention has in particular the advantage of improving thecompensation provided by the existing birefringent films used instructures for compensating for birefringence effects.

[0067] Further advantages and features of the invention will becomeapparent on reading the detailed description which follows and which isgiven with reference to the appended drawings in which:

[0068]FIG. 1 shows the conoscope of an uncompensated TN cell;

[0069]FIGS. 2a and 2 b, the ellipsoids of a positive-type uniaxialmedium and a negative-type uniaxial medium, respectively;

[0070]FIG. 3 shows schematically a structure of a compensated cell;

[0071]FIG. 4 shows schematically the structure of the Fuji-type film andFIG. 5 a conoscope obtained for a liquid-crystal cell compensated usingsuch a film;

[0072]FIG. 6 depicts a first scheme for a cell compensated using astructure according to the invention;

[0073]FIG. 7 is a second scheme for a cell compensated according to theinvention;

[0074]FIG. 8 is an exploded view of the structure depicted in FIG. 6,comprising a Fuji film and a negative uniaxial medium in the holographicplane, and

[0075]FIG. 9 is the conoscope obtained by this structure;

[0076]FIG. 10 is an exploded view of a structure comprising a Fuji filmand a holographic oblique negative uniaxial medium, and

[0077]FIG. 11 the conoscope obtained by this structure;

[0078]FIG. 12 is an exploded view of a structure according to theinvention comprising a Fuji film having an improved CTA and aholographic oblique negative uniaxial film, and

[0079]FIG. 13 the conoscope obtained by this structure.

[0080] It has been discovered that combining a volume hologram havingjudiciously determined parameters with a film of inclined optical axisor oblique axis, such as the Fuji film, considerably improve thecompensatability of the film. In addition, optimizing the CTA of theFuji film also improves the conoscope obtained with the Fuji/hologramcompensation structure.

[0081]FIG. 6 shows in a simplified manner an example of such aconstruction. The orientation of the device is made with respect to theEast-West and North-South directions indicated at the bottom of thefigure.

[0082] The liquid-crystal screen 20 comprises, for example, a twistednematic liquid crystal. This liquid crystal is sandwiched between twoglass plates 21 and 22 (not shown in this figure), the faces of theplates that are in contact with the liquid crystal having been treatedby rubbing so as to define the orientation of the molecules in contactwith these faces and their tilt with respect to the plane of the faces.

[0083] The liquid crystal is placed between a first polarizer 23, avolume hologram 24, a first film 25, for example of inclined opticalaxis, sold by Fuji and a second film 26, substantially identical, forexample, to the first film 25, a volume hologram 27 and a secondpolarizer 28. The two polarizers are oriented, for example, at 90° toeach other, possibly to within a few degrees thereof.

[0084] The invention also applies to all the compensation filmscomprising at least one birefringent medium whose optical axes areoblique (with respect to the plane of the liquid-crystal screen) orinclined in the plane of the film, for example films of inclined opticalaxis of the polymerized nematic liquid-crystal type that are currentlynot commercialized.

[0085] Holographic Film or Negative Uniaxial Film

[0086] The volume hologram is a holographic film in which an indexgrating has been recorded in the volume. In such a film, the opticalaxis is coincident with the normal to the plane of the index layers.

[0087] The characteristics of this holographic film are defined so as inparticular to improve the compensatability of the film of inclinedoptical axis.

[0088] The holographic film is equivalent to a negative uniaxial mediumand operates in form birefringence mode. It possesses artificialbirefringence properties in the case of wavelengths much longer than thespacing of the layers forming the grating. The sinusoidally modulatedindex layers are spaced apart with a smaller spacing than the wavelengthwhich passes through them. These layers constitute nondiffractingholograms for the light used for the liquid crystal.

[0089] The fringes may be created by ultraviolet light interference in aphotosensitive material. The recording process used is, for example,described in the Applicant's patent FR 2 778 000.

[0090] Retardation of the Film

[0091] The equivalent retardation R_(h) of the holographic grating as afunction of the modulation of its refractive index Δn is obtained, forexample, from the following simple formula:$R_{o} = {{- \frac{\Delta \quad n^{2}}{n_{o}}} \times d}$

[0092] with

[0093] d: thickness of the film

[0094] n_(o): mean index

[0095] Δn: modulation of the refractive index.

[0096] For example, a typical value for Δn for producing a holographicgrating of a photopolymer manufactured by DuPont is equal to 0.045.Taking a typical film thickness of 25 μm and a mean index n_(o)>1.5,gives a retardation value R_(h) of −40 nm. The retardation is calculatedusing the method described for example in the publication entitled“TN-LCD viewing angle compensation with holographic gratings” by C.Joubert et al., Photonic West '99, SPIE Proceedings No. 3635, 137-142(1999).

[0097] Each film may have a retardation of less than −150 nm andpreferably falling within the [−10 nm to −100 nm] range of values.

[0098] Angles

[0099] The orientation of the optical axis of the holographic film liesin the plane of the Fuji film or is tilted with respect to the plane ofthis film. The angle of orientation of the index layers is definedaccording to the characteristics of the film.

[0100] The angle of inclination of the optical axis of a holographicfilm can make an angle θ of between 0° and 90° with the normal to theplane of this film.

[0101] The projection of the optical axis of the holographic films onthe plane of the film makes an angle φ of between 0° and 360° forexample.

[0102] The angles θ and φ associated with each holographic film will beoptimized by simulation, in order to produce an effective compensator.

[0103] The parameters that it is desired to optimize are especially:

[0104] the retardation Rp of the “in plane” negative uniaxial film foran angle θ of approximately 0° (optical axis perpendicular to the planeof the screen);

[0105] three values (R_(F), θ_(F), φ_(F)) corresponding, respectively,to the retardation of the oblique negative uniaxial film produced byholography, to the angle θ_(F) of its optical axis with respect to thenormal to the screen, and to the angle φ_(F) of the projection of theoptical axis in the plane of the screen.

[0106] If the compensator comprises several holographic films, eachassociated trio (R_(F), θ_(F), φ_(F)) must be optimized when taking theentire application into account.

[0107] The compensator is optimized by using, for example, a programwith the name DINOS sold by Autronic-Melchers GmbH capable of modelingthe optical transmission of a multilayer comprising a liquid-crystalcell, a Fuji film including a CTA, and a holographic film.

[0108] The optimization is accomplished, for example, by displaying thecontrast conoscope obtained for a given configuration and by looking forthat variation in the compensation films that will improve the viewingangle. The final solution—a better compensation structure—is obtained bysuccessive approximations and iterations.

[0109] The holographic assembly may include one or more films eachhaving a multilayer grating, each layer having its own orientation.

[0110] It is also possible to modify the CTA of negative uniaxial axisin the plane of the Fuji film. The modification consists, for example,in stretching the existing CTA for the commercial Fuji film so as tovary the value of the retardation, depending on the compensation to beapplied to the Fuji film. This variation is introduced into theoptimization steps described above.

[0111]FIG. 7 shows an alternative embodiment of FIG. 6, in which aholograph 24, 27 is bonded to each of the faces of the liquid-crystalcell 20, and the Fuji film 25, 26 is placed between a polarizer 23, 28and a hologram 24, 27.

[0112]FIG. 8 shows an exploded view of the diagram of FIG. 6, comprisinga non-uncrossed Fuji film combined with a negative uniaxial film havinga retardation value of −25 nm.

[0113] The liquid-crystal screen 30 comprises, for example, a twistednematic liquid crystal. This liquid crystal is sandwiched between twoglass plates 31, 32 whose faces in contact with the liquid crystal havebeen treated by rubbing so as to define the orientation of the moleculesin contact with these faces and their tilt with respect to the plane ofthe faces. The direction of rubbing on the face 31 is at −45° to theWest-East direction. The direction of rubbing on the face 32 is at +45°to the same direction. Depending on the configuration, the polarizer 33associated with the face 31 is oriented at 90° to the direction ofrubbing on this face. The polarizer and the analyzer are thereforeoriented at 90° to each other, possibly to within a few degrees thereof.

[0114] The analyzer 34 associated with the face 32 is oriented at 90° tothe direction of rubbing on the face.

[0115] The compensation structure bonded to the face 31 comprises, forexample, a first holographic film 35 (a negative uniaxial film) having aretardation value of around −25 nm, angles θ and φ having values of 90°and 135° respectively, a Fuji film 36 as described above, having anangle φ with a value of 315° and its CTA 37 having a retardation valueof −80 nm and an angle θ of 0.

[0116] The compensation structure bonded to the face 32 comprises, forexample, a second holographic film 38 having a retardation value ofaround −25 nm, angles θ and φ having values of 90° and 45° respectively,and a Fuji film 39 as described above having an angle φ with a value of225° and its CTA 40 having a retardation value of −80 nm and an angle θof 0.

[0117]FIG. 9 shows the isocontrast curves of the compensated celldescribed in FIG. 8. The conoscope shows an improvement in thecompensatability compared with the conoscope of FIG. 4, representativeof the commercial Fuji film.

[0118]FIG. 10 is an exploded view of an example of a structure accordingto the invention in which the compensatabilities of the commercial Fujifilm are improved using an oblique negative uniaxial film having aretardation of −25 nm.

[0119] Compared with FIG. 8, only the characteristics of the films 38and 35 have changed.

[0120] The compensation structure bonded to the face 31 comprises, forexample, a first holographic film 35 having a retardation value ofaround −25 nm, angles θ and φ having values of 120° and 135°respectively, a Fuji film 36 as described above having an angle φ equalto 315° and its CTA 37 having a retardation value of −80 nm and an angleθ of 0.

[0121] The compensation structure bonded to the face 32 comprises, forexample, a second holographic film 38 having a retardation value ofaround −25 nm, angles θ and φ having values of 60° and 45° respectively,and a Fuji film 39 as described above having an angle p with a value of225° C. and its CTA 40 having a retardation value of −80 nm and an angleθ of 0.

[0122]FIG. 11 shows the isocontrast curves of the compensated celldescribed in FIG. 10. Note there is an improvement over the conoscopesobtained in FIGS. 4 and 8.

[0123]FIG. 12 shows a variant in which the CTA of the Fuji film isimproved, for example by modifying the value of its retardation.

[0124]FIG. 12 is an exploded view of a compensated liquid-crystal cellwhich differs from the structure described in FIG. 10 by the CTA.

[0125] The CTA of the Fuji film in this example, labeled 37 and 40, hasa retardation value of −110 nm.

[0126] The isocontrast curves obtained with such a structuring and givenin FIG. 3 show an improvement over the conoscopes obtained above.

[0127] Various arrangements of the holographic film and of the film tobe improved may be imagined without departing from the scope of theinvention, some of which are given in the table below as an illustrationbut in no way implying any limitation. Position of Fuji film Position ofthe hologram 1 The Fuji film is placed The hologram is placed between aface of the between the polarizer and liquid-crystal cell and the Fujifilm a face of a hologram 2 The Fuji film is placed The hologram isplaced between the polarizer between the liquid-crystal and the hologramcell and the Fuji film

[0128] Various arrangements of the compensation half-structures 1 and 2are given in the table.

[0129] The liquid-crystal cell may be “flanked” on each side by a type-1or type-2 compensation structure.

[0130] It may also face the 1-type structure on one side and the 2-typestructure on the other side.

[0131] Without departing from the scope of the invention, theliquid-crystal cell may be of the twisted nematic (TN) type.

1. A display device comprising a liquid-crystal cell element (20) placedbetween two polarizers (23, 28) comprising at least one opticalstructure for compensating for the variations in birefringence of saidliquid crystal according to the viewing angle, characterized in thatsaid optical compensation structure comprises at least one oblique-axisfilm (25, 26) of polymerized liquid-crystal type suitable for at leastpartly compensating for the undesirable effects of the naturalbirefringence of the liquid crystal, combined with a volume hologram(24, 27) of small retardation suitable for improving thecompensatability of said film.
 2. The device as claimed in claim 1,characterized in that said oblique-axis film is of the nematic orpolymerized discotic liquid-crystal type.
 3. The device as claimed ineither of claims 1 and 2, characterized in that said hologram is aholographic film having an optical axis in the plane of the oblique-axisfilm.
 4. The device as claimed in either of claims 1 and 2,characterized in that said hologram is a holographic film having anoptical axis tilted with respect to the plane of this film.
 5. Thedevice as claimed in one of claims 1 to 4, characterized in that thevalue of the retardation of said volume hologram is less than −150 nm,preferably between −10 and −100 nm.
 6. The device as claimed in one ofclaims 1 to 5, characterized in that the holographic assembly consistsof at least two holographic films each comprising a multilayer gratingthe layers of which have their own orientation.
 7. The device as claimedin one of claims 1 to 6, characterized in that a first face of theliquid-crystal cell, being intended to lie on the same side as anobserver, is bonded to a first oblique-axis film which is itself bondedto a hologram and in that a second face of the liquid-crystal cell isbonded to a second oblique-axis film which is itself bonded to ahologram.
 8. The device as claimed in one of claims 1 to 6,characterized in that a first face of the liquid-crystal cell, beingintended to lie on the same side as an observer, is bonded to a firsthologram which is itself bonded to a first oblique-axis film itself andin that a second face of the liquid-crystal cell is bonded to a secondoblique-axis film which is itself bonded to a hologram.
 9. The device asclaimed in one of claims 1 to 6, characterized in that a first face ofthe liquid-crystal cell, being intended to lie on the same side as anobserver, is bonded to a first hologram which is itself bonded to afirst oblique-axis film itself and in that a second face of theliquid-crystal cell is bonded to a second hologram which is itselfbonded to a second oblique-axis film.
 10. The device as claimed in oneof claims 1 to 9, characterized in that said oblique-axis film comprisesa stretched plastic film of the CTA type having a negative uniaxialbirefringence and in that the retardation value of this film is tailoredto the holographic film and to the oblique-axis film.
 11. The device asclaimed in one of the preceding claims, characterized in that theliquid-crystal cell is of twisted nematic type.
 12. The use of thedevice as claimed in one of claims 1 to 11 to compensate for thebirefringence effects in display devices, such as microcomputer screens.